OEM Whitepaper: Background to DEQX-HD™ When is HD-Audio going to happen? As video’s transition from analogue to digital HD powers ahead, consumer audio is only starting its conversion to HD. Although most recording and production moved from analogue tape to 24/96 digital HD production years ago, the distribution of HD music and movie soundtracks is only just beginning with 24/96 stereo downloads and Blu-ray discs carrying Dolby true HD™ and DTS-HD Master Audio™.

However, as 99% of consumers have limited funds, HD-media is only as good as the resolution of the playback system, and consumer loudspeakers are blocking widespread acceptance of HD-audio. As Floyd O’Toole writes in Sound Reproduction: Speakers and Rooms: "The biggest change in audio has been the integration of digital processing into virtually everything—except loudspeakers. With only rare exceptions, the product has stubbornly resisted the logical next step...perhaps it will change when the current generation grows up.''

Fortunately we don’t have to wait that long. Arguably, passive speaker design reached the point of diminishing return decades ago; perhaps at about the time pro-audio production moved from passive to active speakers. For consumers though, the convenience of passive speakers has been hard to beat when ‘going active’, in itself, doesn’t provide the sort of quantum leap in sonic realism that HD offers for video.

For video it was simple: just multiply the number of pixels and the data rate by about a factor of three! Interestingly, as far as HD-audio media is concerned, the de-facto 24/96 digital standard also happens to represent a three-fold data increase over CD (stereo cd=1.4Mb; stereo 24/96=4.6Mb/sec). But, without also improving the playback environment by about a factor of three, most consumers can’t appreciate what 24/96 HD has to offer.

Of course, by improving their playback system to HD resolution, they will also marvel at what has lain dormant on their existing CD collection.

Unlike consumer audio electronics, the mechanical behaviour of affordable passive loudspeakers has limited typical performance to:
  • About plus/minus 2-3dB of frequency-response accuracy
  • 1% to 2% THD (measured charitably using swept monotonic tones)
  • Alarming levels of Intermodulation/break-up distortion
  • Noticeably incoherent timing/group-delay errors
  • Significant crossover distortion and even driver time-misalignment
  • Sound ‘compressed’ playing loud complex content
  • Uneven frequency dispersion
  • Electronic IM distortion caused by passive crossovers
  • Uncompensated impedance and back EMF effects on amplifiers
  • Encourages amplifier IM distortion due to wide power bandwidth
  • Inability to compensate for typical room acoustics

Introducing DEQX-HD™
DSP has been used for many years with varying success in high-end and professional speaker designs. But there’s DSP and DSP, and almost all DSP used today for loudspeaker active crossovers are still based on tradition non-linear phase crossover filters combined with simple driver dales and bi-quad parametric EQ.

DEQX has a three-decade legacy that includes pioneering DSP pro-audio products and today’s PC-based music production . From this pioneering legacy, DEQX-HD™ introduces a genuinely new generation of DSP software that, combined with an active architecture, addresses all of the issues mentioned above that are normally outside of hands of speaker designers. Only state-of-the-art DSP combined with state-of-the-art speaker cabinet and driver design pave the way for affordable mainstream HD loudspeakers.

There have been several attempts to make major improvements to consumer speakers, most notable being attempts to introduce various forms of active speakers, which have long been essential for pro-audio. Unfortunately the improvements offered by going active alone, however helpful, have not been the quantum improvement necessary to justify a change in the way audio components were marketed. This situation has changed more recently with integrated and ‘lifestyle’ solutions that include at least semi-active designs, while active subwoofers have simplified delivering low frequencies. Apart from its DSP platform, DEQX-HD™ usually requires a 3-way active architecture although powered ‘HD-subwoofers’ often deliver bass. This can limit the added requirement for amplifiers to just one low power amp for each tweeter.

The transition to digital audio is already changing the way consumers obtain and listen to music, which challenges the long term relevance of traditional Hi-Fi separates. The use of hard disc playback on car and home PC entertainment servers means that the growing availability of HD 24/96 digital can help revive interest in sonic realism that the “good enough for rock and roll” offerings of CD and passive speakers have disallowed for the mainstream. Click for HDP-3 OEM information sheet.

How to implement DEQX-HD™
DEQX recently announced the option for OEMs to license DEQX-HD™ for active speaker implementation. Licensing offers OEMs the best possible cost effectiveness for DEQX implementation, and allows several options for hardware production and control of supply. DEQX provides its DSP and analogue/digital reference I/O reference designs that OEMs can use as the basis of their own hardware designs, allowing custom analogue and digital I/O and control interface designs. DEQX also provides working generic and custom DSP+I/O modules, or OEMs can license generic or custom designs and arrange their own production. DEQX can optionally manage production contracting on your behalf using DEQX’s experienced contract assembly and test facilities. The unique offerings of DEQX-HD™ speaker processing:
While consumer loudspeakers have remained stuck in their passive regime and minor improvements have become increasingly expensive to deliver, it has been left to ‘audiophiles’ like expert wine tasters, to notice what exactly sounding better if at all, and by how much. Mainstream consumers on the other hand can have trouble noticing significant differences between typical consumer speakers because very few are close to sounding actually ‘real’, and only ‘real’ is what consumers can easily identify with. DEQX-HD™ is intended to offer the last of the elusive tools necessary to allow reasonably affordable speakers to sound ‘real’ to typical consumers. Fortunately, typical audio consumers and not just audiophiles notice a major improvement in sonic realism when HD media is combined with a (roughly) three-fold improvement in key loudspeaker performance criteria typically as follows:

  Hi-fi speakers HD speakers
Frequency response 40Hz - 20kHz +/- 3dB +/- 0.5dB
Group-delay errors 60Hz - 20kHz Significant Insignificant
Distortion (not includ. crossover) >1-3% <0.5%
Crossover distortion Significant Insignificant
Dispersion variation at HF crossover >4dB <1dB
InterModulation/break-up distortion Significant Much less
Room correction No Yes

Combined with these benefits are the advantages that an active regime has always provided. These include less amplifier Intermodulation distortion due to each amplifiers roughly one-third reduction in frequency spectrum responsibilities, allowing each amp to be targeted for ideal performance over only 3 to 4 octaves. This is the single most important issue normally effecting audiophile amplifier complexity and cost.

DEQX-HD™ provides a suite of initiatives to help solve a multitude of problems at minimal cost. DEQX’s patented low-latency phase and amplitude compensation is combined with a new generation active regime that uses linear-phase crossovers that minimize linear and non-linear errors in the frequency-dependent phase and amplitude domain for each driver type, such as subwoofer/s, bass, midrange and tweeter.

To provide the necessary quantum leap in audio realism that ‘HD-Audio’ implies, DEQX-HD™ speakers can perform more realistically than traditional active speakers. DEQX-HD™ processing combined with affordable high-performance limited bandwidth (active) amplifiers allow a performance revival in all sectors of the mainstream audio markets.

DEQX-HD™ speakers have frequency-related phase and amplitude compensation as well as active crossovers and amplifiers either built-in or supplied in an external driving unit that is bi-wired or tri-wired to the speaker. In the transition to HD-Audio an ‘HD-ready’ loudspeaker might include by-passable internal passive crossovers to allow its use in traditional Hi-Fi, whereby an additional active unit could be sold as the ‘HD’ upgrade. A typical DEQX-HD™ main speakers would be a 3-way, or 2-way plus one or two powered subwoofers.

DEQX™ phase/amplitude correction can also be used to correct passive speakers in cases where it is impractical to bypass the internal crossovers. In this case the processing provides detailed compensation of group-delay and frequency-response from pseudo-anechoic measurements, from which the time and frequency domain correction is automatically determined. Mono or stereo subwoofers can optionally be added using time alignment control that can include measuring, viewing and adjusting the impulse response of room measurements for both subwoofers and main speakers. Room correction can be automatically and manually applied from room measurements. Multi-band parametric EQ allows real-time manual tuning and saved presets.

DEQX-HD™ processing is a 2-way or 3-way active implementation of DEQX™ processing. Group-delay and frequency-response correction are applied to each driver type, the factory determined correction is then integrated with steep(er) linear-phase crossovers to minimize crossover, dynamic and break-up distortion, and improve dispersion by minimizing driver beaming. Room correction of group-delay and frequency-response can be applied from room measurements, while multi-band parametric EQ allows real-time manual tuning and saved presets.

DEQX™ (correction of passive speakers) addresses the following:
Phase and group-delay correction: Provides typically 3ms window of anechoic correction of phase/group-delay. This essentially delays the more ‘on-time’ frequency groups by up (by typically 3ms) to allow slower frequencies to catch up, providing the originally recorded sonic coherence. Frequency-response correction: Provides on-axis accuracy typically within 1dB (plus/minus 0.5dB) determined by measurement accuracy. 24-bit 96kHz processing provides bandwidth to 40kHz. Better bass delivery into room: Optionally provides properly time integrated boundary/corner-located subwoofer/s to facilitate higher efficiency and coherent bass delivery into typical rooms. This implies a high-pass filter to the main speakers that typically reduces mid/bass driver distortion. Room correction: Provides automated and manual EQ tuning using FIR-based subwoofer correction, and/or real-time parametric EQ. DEQX-HD™ active configurations provides these additional features:
Reduced crossover distortion: Steep linear-phase crossovers to tweeter, typically 48db/oct (300dB/oct max) largely eliminate audible crossover distortion. Reduced congestion/dynamic distortion: Steeper crossovers quarantine affordable drivers to their 3 to 4 octave comfort zone, significantly reducing dynamic distortion, IM distortion and break-up. Steep-filter pre-ringing compensation: Steep crossovers traditionally ring in a way that can render their use undesirable. Patent-pending group delay compensation causes each driver to cancel ringing effects using a pre-ringing phase cancellation function. Inherently increased resolution: Reduced crossover and dynamic distortion is achieved because group-delay correction of mid and tweeter drivers combined with quarantining the output of all driver’s to their most linear operating range, minimizes crossover distortion and maximizes net resolution. Easy low-distortion driver implementation: Affordable drivers that offer unusually low distortion (<0.5%) often do so at the expense of an accurate frequency-response and absence of resonances as is traditionally required for passive designs. Such drivers however are easily implemented using DEQX because frequency-response errors, timing errors and resonances are dealt with in detail. Improved natural dispersion: Sub-woofer and mid-driver HF beaming is largely eliminated due to steep low-pass filters, resulting in a more even off-axis response for natural sounding power response. Simple to use DEQX™ and DEQX-HD™ development regime:
DEQX’s wizard driven software connects to DEQX DSP hardware or OEM modules via USB to a windows computer/notebook. The hardware has a phantom powered microphone input for connection to a DEQX™ microphone (supplied optionally).

All measurements, analysis and correction filter generation and system configuration specifications are performed within DEQX’s own integrated software package. Several evaluation and development platforms are available, each being configured with:
  • Four stereo inputs: Analogue RCA + XLR balanced, Digital RCA + XLR bal.
  • Standard stereo 3-way active outputs (6 x RCA)
  • Optional additional stereo balanced active outputs (6 x XLR) OR
  • Optional additional stereo transformer balanced outputs (6 x XLR) OR
  • Optional 3 x S/PDIF RCA + 3 x AES/EBU XLR. This board includes AES sync input that can asynchronously sample rate convert all digital outputs up to 102kHz

Development hardware/software package options:
1) PDC-2.6P Processor/preamp
  • Allows DEQX™ and DEQX-HD™ implementation
  • Four Inputs: Analogue; 2 x RCA 2 x XLR. Digital; 2 x RCA SPDIF, 2 x XLR AES/EBU
  • Six RCA analogue outputs allows up to 3-way DEQX-HD™ implementations
  • DEQX DSP engine
  • USB PC interface
  • Professional analogue audio quality
  • 1U high chassis

    Options:
  • DEQX™ Behringer microphone
  • DEQX™ Earthworks microphone
  • Balanced active outputs (6 x XLR) OR
  • Transformer balanced outputs (6 x XLR) OR
  • Digital outputs (3x XLR AES/EBU, 3 x S/PDIF)
  • Remote control

2) Audiophile HDP3 Processor/preamp: Includes listing as per #1 (above) plus:
  • 2U chassis with linear main power supply
  • Audiophile analogue audio quality
  • Ability to install one balanced analogue out module plus Digital out module

    • Options: As #1 (above) but with the ability to install digital and balanced analogue outputs

Implementing DEQX-HD™ in production: Options for implementing DEQX-HD™ will depend on your system design. While traditional active speaker designs, especially in pro-audio, have active crossover, amplifiers and power supply installed inside the speaker cabinet, this may not be desirable for consumer speakers, especially if a speaker contains passive crossovers that can optionally be bypassed for DEQX-HD™ active operation.

DEQX™ or DEQX-HD™ implementation requires a DEQX DSP engine designed for your application based on DEQX’s standard designs, or as generic DEQX OEM modules that are available with or without digital and analogue input and outputs. DEQX’s NTDSP module for example can be plugged into your custom designed main interface PCB that can contain ADC, DAC/s and/or amplifiers.

The NTDSP module contains one S/PDIF input and outputs as well as the necessary USB interface to PCs for setting up and any customised measurements, as well as RS232 and high-speed I2S serial ports and control ports. One NTDSP module can provide 1 or 2 input and 2 to 6 output channels, providing 2-in 6-out 3-way crossovers or for example a 1-in, 4-way active crossovers. For maximum system economy, digital input power amplifiers such as the TI Equibit range, can be driven gluelessly from the NTDSP card’s I2S outputs to provide cost effective D-class active solutions.

Standard NTDSP module installed in custom designed PCB:
The DEQX NTDSP-module is installed into a custom-designed OEM PCB that provides necessary panel mounted connectors, ADC/DAC converters, other I/O and PSU. The plug-in DEQX NTDSP-module is a multilayer PCB with standard type of SIMM (memory) connector. DEQX can supply custom PCB I/O designs that contain the required ADC and DAC converters, and/or digital receivers, power supplies etc. One or more NTDSP cards can plug into the custom board.

The DEQX NTDSP-module contains:
  • Analog Devices SHARC ADSP-21369 DSP
  • Minimum 8MB x SDRAM for FIR filtering - max 64MB
  • Minimum 8MB FLASH memory for main program and filter storage - max 64MB
  • USB port for optional PC interface and/or factory set up programming
  • RS232 port for optional interface to automation systems or other DEQX processors
  • Multiple 12S input and output ports for ADC/DAC and digital interfaces
  • S/PDIF input and output interface
  • On board mini switching power supply converting 3.3V to 1.3V for DSP
  • Power requirement 3.3V @ 1A supply
  • JTAG port for PCB production programming and testing

The third generation Analog Devices SHARC (Super Harvard ARChitecture) DSP is a 32-bit floating-point processor. DEQX’s software provides typically <0.00001% THD+S/N in the digital domain with FIR and IIR filters running.

The NTDSP-module is capable of the following features:
  1. Provide 2nd to 8th order low-pass filter (Linkw/Bwth) to subwoofer amplifier.
  2. Provide 2nd to 8th order high-pass filter (linkw/Bwth) to L/R main speakers (line level).
  3. Provide automated group-delay and frequency-response correction to subwoofer from room measurement. Requires PC plug in for set-up unless custom controller.
  4. Provide automatic and manual parametric EQ correction (minimum-phase frequency-response correction) using a customised interface without requiring connection to an external PC.
  5. Allow DEQX™ or DEQX-HD™ correction of external main speakers using generic selection of correction filters pre-loaded into its internal FLASH memory. An optional customised user interface can provide additional features such as user defined EQ and room measurement.
  6. Allow pseudo-anechoic measurement and correction of any main speaker or in-wall speakers using external PC for set up (requires trained installer/dealer).

DEQX™ and DEQX-HD™ Licensing requirements:
It is a requirement that products providing DEQX™ or DEQX-HD™ performance contain the approved DEQX logos on the product and in published product descriptions. It is also a requirement that mention of DEQX™ or DEQX-HD™ as relevant, be included in promotional and advertising materials.

Custom designed PCB for I/O and NTDSP installation
Hardware development: Customised PCB designs can be developed by DEQX according to I/O requirements and specification. The cost of the main board design depends on requirements but for ADC/DACs. Either DEQX or OEMs can arrange manufacturing of the board. DEQX does not require royalty payments for custom board designs or use of software. All royalties and licensing fees are included in the cost of the NTDSP module. In the event of a high quantity production, a licensing arrangement allows the most cost effective option. Licensing arrangements includes an establishment fee and royalty per speaker basis. In this case the OEM would manufacture custom hardware designs including the DSP engine or DEQX can arrange manufacturing on the OEM’s behalf.

Software development: DEQX’s standard software is included with OEM modules and licensing arrangements as agreed with the OEM. DEQX’s Rev-3 software, due to be released in Q1 2010, allows customised OEM interfaces that provide specifically required functionality and appearance as specified by the OEM to control lower level standard DEQX operational features invisibly to the user or installer.

Depending on OEM’s requirements, room measurement and correction can be automated and contained within the main custom PCB. For final tuning of the system to the user’s personal preferences, it is an option to provide a remote control for manual tuning of the room from listening position/s. A remote control, or control via a home automation server using RS232, allows (presently) up to four different preference settings to be chosen.

For demanding installations, where a trained installer is available, or users are prepared to use a PC for set-up, a portable PC notebook running DEQX’s full suite of software has the advantage that final real-time tuning of a system is possible from any point in the room. More complex forms of measurement and correction are possible as determined by the OEM for user access.

All loudspeaker models would have previously been measured and filter sets stored on the NTDSP-module’s FLASH memory for easy selection without the need for PC connection. This memory can be updated by connecting the USB port to a PC and downloading updated software from OEM’s website. It is not necessary to measure each loudspeaker from a production batch (although this is an option for optimum results) as generic correction for a given speaker model is usually all that’s required.

Some issues that arise from the feed-through facility that might be considered are:
  1. The transparency of the stereo ADC and DAC converters is of paramount importance in high-end systems. It is an option to allow the ADC and DAC circuitry to be fully balanced for ultimate ADC and DAC transparency.
  2. The subwoofer may be some distance from the Pre-amp and power amplifiers so it may be desirable to offer balanced stereo inputs and outputs (in addition to unbalanced RCA), to allow long line-level cable runs.
  3. Providing a remote control (IR) that allows tuning the room from listening position/s.
  4. A remote control can also be used to allow real-time ‘tone control’ to compensate for different audio media and film soundtracks. The current PDC-2.6 and HDP3 offers 100-memories that can store 3-band EQ for instant recall for particular recordings. Tone controls can include parametric EQ, which is normally not provided by high-end preamps or remote controls. These features can be provided by an automated home server (e.g. Crestron or AMX) if an RS232 connector is provided. Note that the NTDSP-module provides RS232 interface hardware.

Implementing DEQX-HD™ processing for main speakers
The processor described above, in relation to the sub-woofer controller, is automatically capable of providing DEQX™ processing where it also intercepts the main speaker’s signal; at least for adding HF filtering and possibly delay. DEQX-HD™ provides the same correction and EQ capabilities as DEQX™ but with the addition of fully active crossover filtering using DEQX’s steeper linear-phase crossovers with integrated driver group-delay and frequency response correction.

The normal scenario is for this to be a 2-way active system plus stereo subwoofers. However, the NTDSP-module also allows custom 3-way plus stereo subwoofer configurations which introduces many other options for a new generation of truly “HD” loudspeaker designs, including the following features:
  1. Each driver has its own amplifier that allows less than heroic full-bandwidth audiophile amplifiers to provide audiophile quality because they each typically only deal with about one-third of the audio spectrum. This also allows different amplifier types to be selected for their ideal operating frequency range.
  2. DEQX-HD™ linear crossover filters utilize DEQX’s patented low latency FIR/IIR filtering. This provides real-time group-delay and frequency-response compensation in fine detail over the full spectrum.
  3. Steep linear-phase filters in the order of 10th order are computed based on anechoic measurements of driver’s phase at the crossover frequency. This sometimes may be 180 degrees out for phase from physical +/- connections, because physical time alignment between drivers is not necessary; it is automatically computed and compensated when drivers are measured in-situ.
  4. Steep filters allow dramatically reduced HF beaming from mid driver for more natural polar and horizontal dispersion across crossover regions.
  5. Steep filters quarantine bass energy and therefore driver excursion into non-linear operating regions, reducing distortion congestion for bass/mid and tweeter drivers at high volume.

Physical implementation of DEQX-HD™
Traditional active speakers, such as used for pro-audio monitoring have active crossovers and amplifiers installed in each speaker for logistical reasons. Those offering digital inputs such as AES/EBU may have all processing in one speaker, which outputs a processed line level signal to a ‘slave’ version of the same speaker. For consumer audio, a passive bi-wire or tri-wire speaker may be more desirable for high end used so that amplifiers can be chosen as required. In the case where a speaker manufacturer does not typically provide a complete solution that includes amplifiers, it is possible to use OEM amplifiers, or to use the ‘optionally passive’ approach, were a speaker does have a passive crossover installed, but that a way is provided to bypass the internal passive filters to allow fully, or semi-active bi-or tri-amping.

The next step
DEQX’s PDC-2.6P or HDP3 will demonstrate what your new (and existing) designs are capable of when using DEQX-HD™ processing. Please contact us at sales@deqx.com